The naturally occurring opiate peptides, including the endorphins, have been implicated in a number of age-related and/or age-aggravated disease disease states, such as obesity, decreased motivation, a variety of psychological disorders, sexual dysfunction, memory and learning disorders, and several neuropathogenic states. Recent studies indicate the existence of multiple forms of endorphins in the pituitary, plasma, and brain. There exists, at this time, no data on the variation of the relative or absolute concentrations of these forms with aging or in the nature of their receptors with aging. In the first part of the proposed study, the multiple forms of the endorphins will be studied in vivo in terms of their peripheral clearance, distribution, and metabolism in young and old rats, using specifically radiolabeled endorphins. Since receptor changes frequently occur with aging, the second part of this proposal will characterize central and peripheral endorphin receptors in young and old rats. This will be done with sensitive radioreceptor assays (RRA) and with specific photoaffinity-labeled endorphins. It is anticipated that the above research will contribute significantly to the determination of the changes which occur in the processing and distribution of endorphins and their receptors with aging, which in turn will help to form a basis for the understanding of their physiological role.